Hi! I have a question about decoupling caps in a minimum circuit with Atmega32U4. How much and which value? The datasheet keeps silent about it. I have 0.1µF caps on almost all VCC lines. Some guy told me it is common sense to have these values.
Alright. But then I would assume having 0.1µF on each VCC line. Why is there a "1µF" (not: 0.1µF) on UCVV? I copied this value from Teensy2.0. But that doesn't explain me why "1µF" there?

@joeymorin I see. The teensy2.0 makers used "1µF" instead of the recommended 10µF, because there is another 1µF in the circuit (on UCAP), and by this they reduced the amount of different components to a minimum and spare production costs, right?

@david.prentice Thanks for these rules of thumb. This part of your post is still not clear to me

Look at Joey's post. And the Teensy schematic. You have 1uF as a "reservoir" whether you select 3.3V or 5V. 10uF would be equally appropriate.

Anything that eventually uses the "VCC" derived from UVCC / UCAP will have their own 100nF capacitors.

There is no need to know the maths or the physics. Just copy the proven design.

If you want a layman's explanation. 100nF provides short term current to fast switching circuits. 10uF or 470uF provide longer term current.

Think of a sprinter versus a marathon runner. (just realise that instead of a 10 second sprint you have a 0.000000001 second switching time, instead of 3 hour run, you have a 0.01 second mains ripple). The sprinter probably eats glucose. The marathon runner eats slower-release carbohydrates.

Ye, good layman explanation. I get a glimpse of the logic behind that. I'm using 4 "sprinters" with 0.1µF to compensate leads, and 2 "marathon runners" with 1µF to compensate gaps. Recommended is, to have 0.1µF on every power line, so I rather should have 6 of these sprinters, instead of 4, right?

6 is the recommended number. Depending on how the circuit is constructed (breadboard, single layer circuit board, presence of ground planes, etc.), how (electrically) quiet the environment, and how harsh the temperature range, you may be able to fudge that a bit.